Diagnosis: shell bilateral symmetrical, cylindrical, gradually swelling from the aperture for about two-thirds of the body length to the broadest diameter and then tapering in the last third of the body length to a bluntly or sharply, asymmetrically shaped apex, which outline resembles in lateral view the dorsal fin of a shark. Test usually hyaline, composed of quartz particles and diatom frustules, giving it the appearance of a regular smooth surface. Aperture usually circular. Nucleus ovular, with nucleoli arranged close to the nuclear membrane.

Dimensions: Thomas: length 210-220 µm, aperture diameter 30-50 m;

Ecology: in oligotrophic and mesotrophic ponds, between sphagnum; rare.

Remarks: Dubious species. Similar in general shell shape with D. scalpellum but more slender and slightly smaller in size. The spine is slightly out of the axis. It shows some resemblance with the dorsal fin of a shark. The nucleus is hard to detect between all inclusions, but when the test is compressed, it can be observed. This species also fits the description of D. curvicaulis in shell structure.

You can label these shells as D. smilion, D. curvicaulis or as D. scalpellum. The drawing to the left is from Penard and shows D. scalpellum.